1. Field of the Invention
This invention relates to the chemical treatment of cellulose fibers to impart the fiber with higher hydrophobicity and/or durability. More particularly, this invention relates to cellulose fiber reinforced cement composite materials using sized cellulose fibers, including fiber treatment methods, formulations, methods of manufacture and final products with improved material properties relating to the same.
2. Description of the Related Art
Ordinary Portland cement is the basis for many products used in building and construction, primarily concrete and steel reinforced concrete. Cement has the enormous advantage that it is a hydraulically settable binder, and after setting it is little affected by water, compared to gypsum, wood, wood particle boards, fiberboard, and other common materials used in building products. This is not to say that water has no effect on cement. Some dissolution of chemical components does occur when cement is saturated with fresh water, and these can be transported and re-deposited in different places if the cement is once again dried.
Asbestos Fiber Cement Technology
About 120 years ago, Ludwig Hatschek made the first asbestos reinforced cement products, using a paper-making sieve cylinder machine on which a very dilute slurry of asbestos fibers (up to about 10% by weight of solids) and ordinary Portland cement (about 90% or more) was dewatered, in films of about 0.3 mm, which were then wound up to a desired thickness (typically 6 mm) on a roll, and the resultant cylindrical sheet was cut and flattened to form a flat laminated sheet, which was cut into rectangular pieces of the desired size. These products were then air-cured in the normal cement curing method for about 28 days. The original use was as an artificial roofing slate.
For over 100 years, this form of fiber cement found extensive use for roofing products (slates, and later corrugated sheets), pipe products, and walling products, both external siding (planks and panels), and wet-area lining boards. Asbestos cement was also used in many applications requiring high fire resistance due to the great thermal stability of asbestos. The great advantage of all these products was that they were relative lightweight and that water affected them relatively little, since the high-density asbestos/cement composite is of low porosity and permeability. The disadvantage of these products was that they were brittle and the high-density matrix did not allow nailing, and methods of fixing involved pre-drilled holes.
Although the original Hatschek process (a modified sieve cylinder paper making machine) dominated the bulk of asbestos cement products made, other processes were also used to make specialty products, such as thick sheets (say greater than 10 mm). These processes used the same mixture of asbestos fibers and cement as the Hatschek process. Sometimes process aids are needed in other fabrication processes, for example, extrusion, injection molding, and filter press or flow-on machines.
Two developments occurred around the middle of the last century that have high significance to modern replacements of asbestos based cement composites. The first was that some manufacturers realized that the curing cycle could be considerably reduced, and cost could be lowered, by autoclaving the products. This allowed the replacement of a portion of the cement with fine ground silica, which reacted at autoclave temperatures with the excess lime in the cement to produce calcium silica hydrates similar to the normal cement matrix. Since silica, even when ground, is much cheaper than cement, and since the autoclave curing time is much less than the air cured curing time, this became a common, but by no means universal manufacturing method. A typical formulation would be about 5-10% asbestos fibers, about 30-50% cement, and about 40-60% silica.
The second development was to replace some of the asbestos reinforcing fibers by cellulose fibers from wood or other raw materials. This was not widely adopted except for siding products and wet-area lining sheets. The great advantage of this development was that cellulose fibers are hollow and soft, and the resultant products could be nailed rather than by fixing through pre-drilled holes. The siding and lining products are used on vertical walls, which is a far less demanding environment than roofing. However, cellulose reinforced cement products are more susceptible to water induced damages, compared to asbestos cement products. A typical formulation would be about 3-4% cellulose, about 4-6% asbestos, and either about 90% cement for air cured products, or about 30-50% cement, and about 40-60% silica for autoclaved products.
Asbestos fibers had several advantages. The sieve cylinder machines require fibers that form a network to catch the solid cement (or silica) particles, which are much too small to catch on the sieve itself. Asbestos, although it is an inorganic fiber, can be xe2x80x9crefinedxe2x80x9d into many small tendrils running off a main fiber. Asbestos fibers are strong and stiff, and bond very strongly with the cement matrix. They are stable at high temperatures. They are stable against alkali attack under autoclave conditions. Hence, asbestos reinforced fiber cement products are themselves strong, stiff (also brittle), and could be used in many hostile environments, except highly acidic environments where the cement itself is under chemical attack. The wet/dry cycling that asbestos roofing products were subjected to, often caused a few problems, primarily efflorescence (efflorescence is caused by the dissolution of chemicals inside the products when wet, followed by the deposition of these chemicals on the surfaces of the products when dried). Efflorescence caused aesthetic degradation of roofing products in particular, and many attempts were made to reduce it. Because the matrix of asbestos reinforced roofing products was generally very dense (specific gravity about 1.7), the total amount of water entering the product even when saturated was relatively low, and the products generally had reasonable freeze thaw resistance. If the density was lowered, the products became more workable (for example they could be nailed) but the rate of saturation and the total water absorption increased and the freeze thaw performance decreased.
Alternative Fiber Cement Technologies
In the early 1980""s, the health hazards associated with mining, or being exposed to and inhaling, asbestos fibers started to become a major health concern. Manufacturers of asbestos cement products in the USA, some of Western Europe, and Australia/New Zealand in particular, sought to find a substitute for asbestos fibers for the reinforcement of building and construction products, made on their installed manufacturing base, primarily Hatschek machines. Over a period of twenty years, two viable alternative technologies have emerged, although neither of these has been successful in the full range of asbestos applications.
In Western Europe, the most successful replacement for asbestos has been a combination of PVA fibers (about 2%) and cellulose fibers (about 5%) with primarily about 80% cement. Sometimes 10-30% of inert fillers such as silica or limestone are in the formulation. This product is air-cured, since PVA fibers are, in general, not autoclave stable. It is generally made on a Hatschek machine, followed by a pressing step using a hydraulic press. This compresses the cellulose fibers, and reduces the porosity of the matrix. Since PVA fibers can""t be refined while cellulose can be, in this Western European technology the cellulose fiber functions as a process aid to form the network on the sieve that catches the solid particles in the dewatering step. This product is used primarily for roofing (slates and corrugates). It is usually (but not always) covered with thick organic coatings. The great disadvantage of these products is a very large increase in material and manufacturing process costs. While cellulose is currently a little more than asbestos fibers of $500 a ton, PVA is about $4000 a ton. Thick organic coatings are also expensive, and hydraulic presses are a high cost manufacture step.
In Australia/New Zealand and the USA, the most successful replacement for asbestos has been unbleached cellulose fibers, with about 35% cement, and about 55% fine ground silica, such as described in Australian Patent No. 515151 and U.S. Pat. No. 6,030,447, the entirety of which is hereby incorporated by reference. This product is autoclave cured, as cellulose is fairly stable in autoclaving. It is generally made on a Hatschek machine, and it is not usually pressed. The products are generally for siding (panels and planks), and vertical or horizontal tile backer wet area linings, and as eaves and soffits in-fill panels. The great advantage of these products is that they are very workable, even compared to the asbestos based products, and they are low cost.
However, cellulose fiber cement materials can have performance drawbacks such as lower resistance to water induced damages, higher water permeability, higher water migration ability (also known as wicking) and lower freeze thaw resistance when compared to asbestos cement composite material. These drawbacks are largely due to the presence of water conducting channels and voids in the cellulose fiber lumens and cell walls. The pore spaces in the cellulose fibers can become filled with water when the material is submerged or exposed to rain/condensation for an extended period of time. The porosity of cellulose fibers facilitates water transportation throughout the composite materials and can affect the long-term durability and performance of the material in certain environments. As such, conventional cellulose fibers can cause the material to have a higher saturated mass, poor wet to dry dimensional stability, lower saturated strength, and decreased resistance to water damage.
The high water permeability of the cellulose reinforced cement materials also results in potentially far greater transport of some soluble components within the product. These components can then re-deposit on drying, either externally, causing efflorescence, or internally, in capillary pores of the matrix or fiber. Because the materials are easier to saturate with water, the products also are far more susceptible to freeze/thaw damage. However, for vertical products, or eaves and soffit linings, and for internal linings, none of these water-induced disadvantages are very relevant.
To summarize, the replacement of asbestos in Europe has been largely by air cured fiber cement products, using PVA fibers, and pressed after forming in the green state. The primary problem with this technology is increased material and manufacturing cost. The replacement of asbestos in USA and Australia/New Zealand has been largely by autoclaved fiber cement products, using cellulose fibers, and formed with lower density without pressing. The primary problem with this technology is increased rate, and quantity, of water absorption into the product when wet, and reduced resistance to freeze thaw cycles.
Certain prior art references teach using fibers that are grafted with a silane or silylating coupling agent. However, these references are directed to improving the bonding between the fibers and the cement so as to increase the strength of the composite material. As such, the coupling agents selected contain primarily hydrophilic functional groups with the specific purpose of bonding with both the hydroxyl groups on the fiber surface and the cementitious matrix. In fact, these references teach away from using coupling agents having hydrophobic functional groups as the hydrophobic groups would slightly decrease, rather than increase, the material strength.
For example, U.S. Pat. No. 5,021,093 teaches grafting a silyating agent to the fiber surface so as to improve the strength of the resulting composite material. The silyating agent comprises molecules containing hydrophilic groups on both ends so that one end can bond with hydroxyl groups on the fiber surface and the other end can bond with the cementitious matrix. The silyating agent essentially serves as a coupling agent that connects hydroxyl groups on the fiber surface to the cementitious matrix.
U.S. Pat. No. 4,647,505 teaches applying a chelating agent to a cellulose fiber to reduce fiber swelling in aqueous and alkaline solutions. The fibers are impregnated with a solution of a titanium and/or zirconium chelate compound. The chelate compound, however, does not react upon contact with the fiber, because the fiber is contained in an aqueous medium, and the chelate compounds described in the patent resist hydrolysis at ambient temperatures. Therefore, this patent describes heating the fibers above 100xc2x0 C. to dry the fibers, thereby allowing the reaction to take place. After drying, the chelate compound(s) react with hydroxyl groups on the cellulose fibers to produce cross-linking between the hydroxyl group residues.
As U.S. Pat. No. 4,647,505 is directed primarily to reducing swelling of cellulose fibers, it is not specifically directed to increasing hydrophobicity of the fibers. Moreover, this patent provides an approach to fiber treatment which requires drying of the fibers in order to induce reaction with the cellulose fibers.
Accordingly, what is needed is an efficient method for preventing damage and degradation to a fiber cement building material, particularly due to water and/or other environmental effects. What is also needed are material formulations and products having improved resistance to water and/or environmental degradation.
The preferred embodiments disclose a new technology: chemically treating cellulose fibers to impart the fibers with hydrophobicity and/or durability, and making cellulose fiber reinforced cement composite materials using these chemically treated cellulose fibers. In one preferred embodiment, the cellulose fibers are treated or sized with specialty chemicals that impart the fibers with higher hydrophobicity by partially or completely blocking the hydrophilic groups of the fibers. However, other embodiments for chemically treating the fibers are also disclosed, including loading or filling the void spaces of the fibers with insoluble substances, or treating the fibers with a biocide to prevent microorganism growth or treating the fibers to remove the impurities, etc.
More preferably, in a sized fiber embodiment, several aspects are disclosed, including fiber treatment, formulations, methods of making the composite materials, and final materials and properties. This technology advantageously provides fiber cement building materials with the desirable characteristics of reduced water absorption, reduced rate of water absorption, lower water migration, and lower water permeability. Final products made from these materials have improved freeze-thaw resistance, reduced efflorescence, reduced dissolution and re-deposition of water-soluble matrix components in natural weathering. It is possible, with the right fiber sizing, to improve other product properties, for example, rot and UV resistances, compared to conventional fiber cement products. It has been found, surprisingly, that these improved attributes in water resistance are gained without significant loss in dimensional stability, strain or toughness. Additionally, the use of sized fibers can result in improved physical and mechanical properties of the final product.
More particularly, the preferred embodiments show that by blocking the hydrophilic sites on the inner and outer surfaces of cellulose fibers with sizing agents, an engineered cellulose fiber can be produced that, when used in fiber cement, still has the advantages of regular cellulose of refining, autoclaving, and manufacture without pressing, but the resultant fiber cement material also approaches or exceeds the performance advantages of artificial fibers such as PVA, in terms of the rate and amount of water absorption when used in fiber reinforced cement composite materials. In addition, smaller quantities of fibers may be used, so that the cost of treating the fiber can be offset by the lower usage of the fiber in products, without a significant reduction in the important physical properties of the material, such as strength and toughness.
In particular, the preferred embodiments show that when used in formulations typical of autoclaved cellulose based fiber cement, the rate of water absorption and the amount of water absorption are greatly reduced in the composite product. The tendency to effloresce, or to dissolve and re-deposit chemicals internally and externally to the product, or to suffer freeze/thaw damage, etc., is reduced.
Also, the treated fibers may still be refined to act as a catch medium in the Hatschek process, they may still be autoclaved without excessive fiber degradation, and they make products adequate in strength without pressing. Furthermore, with lower amounts of actual cellulose fiber being used, the preferred embodiments experience no reduction in key physical properties such as strength, stiffness, and moisture movement, and may, in fact, improve some of these properties.
Thus, the use of engineered sized fibers imparts to the composite material these enhanced properties, and therefore constitute an alternative technology that, when fully implemented, has the potential to improve mechanical properties and the workability with the material in building and construction, while improving the durability of the products in various environments including especially those that involve cyclic wetting and drying, freezing and thawing, and exposure to UV and the atmosphere, regardless of the means of manufacture. They are particularly suitable to the Hatschek process that requires a refineable fiber (to catch solid particles) and to the autoclave curing cycle that allows the replacement of cement with fine ground silica, although they may also be of use in the air cured products, in conjunction with PVA, to reduce the necessity of the expensive process pressing.
Accordingly, preferred embodiments of the present invention will solve many of the problems that are associated with regular cellulose fiber reinforced cement composite materials, such as high water permeability, high water absorption, efflorescence, internal water dissolution and re-deposition of materials, and low durability in freeze/thaw weathering environments in comparison with asbestos cement materials, while maintaining or improving some of key mechanical and physical properties. Surprisingly, less cellulose fiber may be required. Moreover, this technology is also beneficial for solving one of the key problems of air cured, PVA reinforced fiber cement, by eliminating the need for the expensive process of hydraulic pressing of the formed xe2x80x9cgreenxe2x80x9d body, to crush the cellulose fibers and reduce water permeability in finished products.
In one aspect of the present invention, a composite building material is provided comprising a cementitious matrix and cellulose fibers incorporated into the cementitious matrix. At least some of the cellulose fibers have surfaces that are at least partially treated with a sizing agent so as to make the surfaces hydrophobic. The sizing agent comprises a hydrophilic functional group and a hydrophobic functional group, wherein the hydrophilic group permanently or temporarily bonds to hydroxyl groups on the fiber surface in the presence of water or an organic solvent in a manner so as to substantially prevent the hydroxyl groups from bonding with water molecules. The hydrophobic group is positioned on the fiber surface and repels water therefrom.
One preferred formulation of a building material made in accordance with this new technology comprises a cementitious binder, preferably Portland cement; an aggregate, preferably silica which may be fine ground if it is to be autoclaved; one or more density modifiers; cellulose fibers, at least some of the cellulose fibers having surfaces that are at least partially treated with a sizing agent so as to make the surfaces hydrophobic; and one or more additives. The sizing agent comprises a hydrophilic functional group and a hydrophobic functional group, wherein the hydrophilic group permanently or temporarily bonds to hydroxyl groups on the fiber surface in the presence of water or an organic solvent in a manner so as to substantially prevent the hydroxyl groups from bonding with water molecules. The hydrophobic group is positioned on the fiber surface and repels water therefrom.
The hydrophilic sites, for example the hydroxyl functional groups, on these fibers are partially or completely blocked with sizing agents to reduce the affinity to water. The sizing agents may comprise organic compounds, inorganic compounds, or combinations thereof. In one embodiment, the sizing agent comprises both hydrophilic and hydrophobic functional groups. Preferably, the hydrophilic groups on the sizing agent bond with the hydroxyl groups on the fiber surface and thus prevent the hydroxyl groups from bonding with water, while the hydrophobic groups on the sizing agent are positioned on the fiber surface to repel water. The sizing agents can comprise about 50% of the dry weight of the cellulose fibers. Most preferably, sizing agents in the sized fibers are approximately 0.01 to 10% of the cellulose fiber weight.
A method of manufacturing a fiber reinforced composite building material using the formulations described constitutes another aspect of the present invention. One preferred method comprises providing cellulose fibers and treating at least a portion of the cellulose fibers with a sizing agent in the presence of water or an organic solvent. The sizing agent comprises a hydrophilic functional group and a hydrophobic functional group. The hydrophilic group chemically bonds to at least some of the hydrophilic sites on inner and outer surfaces of the fibers to form sized fibers. The sizing agent substantially blocks the hydrophilic sites, thereby reducing the fibers"" affinity toward water. The sized fibers are mixed with a cementitious binder and other ingredients to form a fiber cement mixture. The fiber cement mixture is formed into a fiber cement article of a pre-selected shape and size. The fiber cement article is cured so as to form the fiber reinforced composite building material.
Some of the above steps can be omitted or additional steps may be used, depending on the particular application. The step of sizing the fibers preferably comprises treating the fibers with inorganic compounds, organic compounds, or combinations thereof using techniques involving dry spraying or solution treatments, although other methods of applying sizing agents are feasible, such as coating, painting and impregnation. Each of these techniques preferably occurs in the presence of water or an organic solvent. Preferably, the step of mixing the sized fibers with ingredients to form a fiber cement mixture comprises mixing the sized fibers with non-cellulose materials such as a cementitious binder, aggregate, density modifiers, and additives in accordance with preferred formulations described herein. In another embodiment, the sized fibers can also be mixed with conventional untreated cellulose fibers and/or natural inorganic fibers, and/or synthetic fibers along with the other ingredients. The fabrication processes can be any of the existing technologies, such as Hatschek processing, extrusion, and molding.
A fiber reinforced cement composite material made using the formulations and the processes disclosed has a fiber cement matrix where the sized cellulose fibers are incorporated into the matrix. The hydrophilic sites on the surfaces of these sized fibers are partially or completely blocked with sizing agents to reduce the affinity for water. Some residual sizing agents from the treated fibers may also react with the inorganic and organic components of fiber cement matrix, blocking hydrophilic sites inside and outside of the matrix. As a result, the final product will be more hydrophobic.
Application of the sized fibers reduces the water migration by more than about 9 times within a 8 hour test, more than about 15 times within a 24 hour test and about 25 times after a 96 hour test, as compared to an equivalent formulation made without sized fibers. In one embodiment the sized fibers lower the rate of water absorption of the building product by more than about 5% in the first 8 hours of water soaking test, and reduces the net water absorption by about 10% or more after 24 hours of water soaking test. The water permeability rate is reduced by about 20% or more. Moreover, the sized fibers also reduce the efflorescence, a side effect of the water permeation. Use of the fibers treated with specialty chemicals may improve the UV, rot, and freeze-thaw resistances of the final building product.
Preferred embodiments of the present invention are not limited to sized fibers. Accordingly, in another aspect of the present invention, a building material incorporating individualized reinforcing fibers is provided. At least a portion of the fibers are chemically treated in the presence of water or an organic solvent to improve the building material""s resistance to water and/or environmental degradation.
In another aspect, a building material formulation is provided comprising a hydraulic binder and individualized reinforcing fibers. At least a portion of the fibers is chemically treated in the presence of water or an organic solvent to improve the building material""s resistance to water and/or environmental degradation.
In another aspect, a method of manufacturing a building material incorporating reinforcing fibers is provided. At least a portion of the reinforcing fibers are chemically treated in the presence of water or an organic solvent to improve the fiber""s resistance to water and/or environmental degradation. The reinforcing fibers are preferably individualized. The reinforcing fibers are mixed with a hydraulic binder to form a mixture. The mixture is formed into an article of a pre-selected shape and size. The article is cured so as to form the fiber reinforced building material.
Advantageously, the preferred embodiments of the present invention provide fiber reinforced building materials that have reduced water migration, lower water absorption rate, lower water permeability, less efflorescence, less severe dissolution and re-deposition problems, and improved freeze-thaw resistance, as compared with a building material made from an equivalent formulation without sized cellulose fibers. Furthermore, the preferred building materials are dimensionally stable and retain the advantages of cellulose fiber reinforced materials. The building material with sized fibers can be manufactured using conventional processes for fiber cement materials. Less cellulose fibers may be required in making the composite materials. These and other advantages will become more fully apparent from the following description taken in conjunction with the accompanying drawings.